1. Field of the Invention
This invention relates to shaping glass sheets and has special utility for shaping glass sheets into certain cylindrical, conical and other complicated shapes that are used either as components of laminated aircraft transparencies or as molds for press polishing relatively rigid components of plastic composition of said complicated shapes that are incorporated in aircraft transparencies. A glass sheet shaped to a particular configuration produced according to the present invention either forms a press polishing mold for press polishing a transparent plastic component of a laminated transparency, or forms a component of glass to be laminated to other layers of glass and/or plastic material to form a laminated transparency.
The requirements for parts that are shaped for use as elements of aircraft transparencies or for making press polishing molds used to press polish components for laminated aircraft transparencies are not sufficient to warrant the use of high production equipment of the type used to shape windshields, back lights and curved side lights for automobiles. Therefore, parts destined for use in fabricating aircraft transparencies, either directly (as in components to be laminated) or indirectly (as in shaped molds to press polish components), are usually shaped during fabrication by gravity sagging in a kiln.
The shapes of typical aircraft components are such that it is practical to shape glass sheets in an environment having a substantially uniform temperature. It is more practical to heat a glass sheet supported in a kiln by heated gas products introduced into the kiln in spaced relation to the glass sheet supporting position mixed with the atmosphere of the kiln to produce a hot atmosphere of convective heat having more uniform heat than the radiantly heated atmosphere that results from using radiant heat sources such as electrical heaters or localized gas burners that directly face the glass sheet undergoing shaping.
Molds for shaping glass sheets have upward facing shaping surfaces conforming to the shape desired for the glass sheets to be shaped in the kiln. There are two types of bending molds used in kilns. One is the outline type, which supports the glass sheet about its marginal edge only. Such molds are preferred to make relatively uniformly shaped bends of relatively gentle curvature. However, when glass sheets are required to be shaped to non-uniform curvatures, the upward facing shaping surfaces of the molds are required to be at least essentially continuous, so as to provide support for the shaped glass sheet throughout substantially its entire extent so as to enable the glass to sag by gravity to conform throughout its extent more exactly to the shape of the upward facing shaping surface of the mold.
However, since the kilns develop a hot atmosphere by using heated gaseous combustion products, the gas currents in the kiln contain unoxidized carbon particles and also tend to remove refractory particles from the walls of the kiln and deposit some of these dirt and dust particles on the upward facing surface of the essentially continuous gravity sag bending molds. These deposited particles normally remain on the upward facing surface of the mold when the glass sags to conform to the upper mold shaping surface. The portions of the hot glass sheet come into contact with these particles to develop non-uniformities and form defects known in the art as "bulls eyes".
The presence of "bulls eyes" in a component of an aircraft transparency makes it unsuitable for use in laminating to other components to form a laminated transparency. Also, if the shaped glass sheet is used as a mold for press polishing a plastic component of an aircraft transparency, the "bulls eyes" in the glass cause corresponding surface defects in the plastic component press polished with the defective glass sheet. In either event, the optical defects in the laminated transparency that result from the conditions conducive to the formation of "bulls eyes" in the component of the laminated transparency either directly or indirectly causes a defective transparency that must be scrapped.
2. Description of the Prior Art
U.S. Pat. No. 2,377,849 to Binkert and Jendrisak discloses a gravity sagging bending mold having an air passage therethrough. Vacuum is applied by suction through the passage to enable a glass sheet to sag into close conformity with the upwardly facing shaping surface of the bending mold without forming air pockets between the lower surface of the glass and the upper surface of the mold that would tend to cause the glass sheet to deviate from its desired shape. While suction helps conform the shape of the glass sheet to that of the mold shaping surface, it is not capable of removing dust and dirt particles from the mold shaping surface.
U.S. Pat. No. 3,020,677 to White relates to shaping glass sheets by the gravity sag technique by sagging the sheet into supporting relation on an outline shaping surface of an outline type shaping mold. Since it is well known that it is difficult to control sag of a heat-softened glass sheet in its unsupported central area within the outline of the mold, this patent provides containers of water having apertured tops within the outline of the mold and below the mold shaping surface so that steam is applied through the apertured tops to the bottom surface of the glass sheet in certain localized areas where it is desired to control the amount of sag in the glass. This patent uses the flow of fluid to control shape and is not concerned with particle removal because particles are not supported in facing relation to the vision area of bent glass sheets when an outline type mold is used for shaping glass sheets.
U.S. Pat. No. 3,074,258 to Leflet relates to bending glass sheets mounted in pairs on an outline shaping mold. The glass sheets are conveyed through a bending lehr where the top sheet separates from the bottom sheet during the bending cycle. Blasts of gas are applied in the space between the upper and lower glass sheets to remove dust particles that may deposit between the glass sheets when they separate from one another and before the sheets come into closer, more intimate contact with one another at the end of the bending cycle. No provision is made in this patent to avoid local pockets of gas between the bent glass sheets that prevent complete intimate contact between the bent glass sheets throughout their entire extent.
U.S. Pat. No. 3,216,811 to Golightly discloses the technique of applying hot gaseous combustion products against selective portions of outline sag bending molds when the molds are returning along a conveyor path outside the lehr to a glass loading station at the entrance of the lehr. This technique produces selective heating of portions of the outline mold where the glass tends to be broken as a result of chill cracking because of the difference in temperature between the glass and the metal rail of the mold during the shaping operation. While the hot gases used to heat localized portions of the mold may deposit particles on the mold, such particles would not affect the optical properties of the bent glass sheets because outline molds support glass sheets in the marginal edge portions of the latter that surround the vision area.
U.S. Pat. Nos. 3,607,186 and 3,681,043 to Bognar disclose the application of vacuum to the lower portion of a gravity sag mold in combination with different treatments to the upper portion of the glass sheet supported on the upper portion of the mold. The combination of steps is alleged to improve the conformity of the shape of the glass sheet to that of the mold shaping surface. Neither of these patents are concerned with the problems of removing particles from the mold shaping surface.
U.S. Pat. No. 3,960,535 to Hamilton and Oelke discloses the application of gas from burners to locally condition glass sheets for tempering while at a shaping station. The areas of heat application are in the vicinity of openings through the thickness of the sheets.
U.S. Pat. No. 3,992,181 to Frank discloses the application of cooling fluid to the lower surface of a glass sheet undergoing shaping while moving through a roll forming station. The cooling fluid is so applied to reduce the tendency of the lower forming rolls, which engage hot moving glass sheets for a greater proportion of the time needed for a production cycle than the upper forming rolls, to become hotter than the upper forming rolls during a prolonged, continuous production run.